Registration in android Studio

I am creating an app on android and at the moment because my knowledge is really basic, I have already created several things, but I have an inconvenience, it turns out that I am doing the user registration and what happens is that I wanted to validate if a mail was already registered and I already achieved it only now when it registers a new user error in the app and despite everything if you register

annex my code

    final String password = mcontraseña.getText().toString().trim();
    final String confirm = mconfirma.getText().toString().trim();
        final String name = mNombre.getText().toString().trim();
        final String ciudad = mciudad.getText().toString().trim();
        final String direccion = mdireccion.getText().toString().trim();
        final String documento = mdocumento.getText().toString().trim();
        final String numero = mnumero.getText().toString().trim();
        final String departamento = mdepartamento.getText().toString().trim();

        final String email = mEmailFiedl.getText().toString().trim();


        mAuth2.child("users").addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot snapshot) {
                final boolean[] flag = {false};
                for (DataSnapshot IDGenerado : snapshot.getChildren()) {
                    String usernameTaken = IDGenerado.child("correo").getValue(String.class);
                    if (email.equals(usernameTaken.toLowerCase())) {
                        flag[0] = true; // Indica que ya fue tomado el username
                    }
                }
                // si el flag nunca se marco como verdadero, entonces no se ha tomado el username
                if (flag[0] != false) {

                    mEmailFiedl.setError("Este correo ya se encuentra Registrado");

                } else {
                    if (Objects.equals(password, confirm)) {
                        if (TextUtils.isEmpty(email)) {
                            Toast.makeText(getApplicationContext(), "Ingresa Tu correo!", Toast.LENGTH_SHORT).show();
                            return;
                        }

                        if (!validarEmail(email)) {
                            mEmailFiedl.setError("Email no válido");
                            return;
                        }

                        if (TextUtils.isEmpty(password)) {
                            Toast.makeText(getApplicationContext(), "No Has Ingresado una contraseña!", Toast.LENGTH_SHORT).show();
                            return;
                        }

                        if (password.length() < 6) {
                            Toast.makeText(getApplicationContext(), "La Contraseña Debe ser mayor De 6 caracteres!", Toast.LENGTH_SHORT).show();
                            return;
                        }


                        mAuth.createUserWithEmailAndPassword(email, password)
                                .addOnCompleteListener(registro.this, new OnCompleteListener<AuthResult>() {

                                    @Override
                                    public void onComplete(@NonNull Task<AuthResult> task) {
                                        Toast.makeText(registro.this, "Tu Usuario Ha sido creado", Toast.LENGTH_SHORT).show();
                                        DatabaseReference mDatabase = FirebaseDatabase.getInstance().getReference().child("users");
                                        DatabaseReference currentUserDB = mDatabase.child(mAuth.getCurrentUser().getUid());
                                        currentUserDB.child("Nombre").setValue(name);
                                        currentUserDB.child("ciudad").setValue(ciudad);
                                        currentUserDB.child("correo").setValue(email);
                                        currentUserDB.child("documento").setValue(documento);
                                        currentUserDB.child("numero").setValue(numero);
                                        currentUserDB.child("direccion").setValue(direccion);
                                        currentUserDB.child("departamento").setValue(departamento);
                                        currentUserDB.child("image").setValue("default");

                                        if (!task.isSuccessful()) {
                                            Toast.makeText(registro.this, "Error no hemos podido registrarte" + task.getException(),
                                                    Toast.LENGTH_SHORT).show();
                                        } else {
                                            startActivity(new Intent(registro.this, principal.class));
                                            finish();
                                        }
                                    }


                                });


                    } else {
                        mconfirma.setError("Las Contraseñas no son iguales");
                    }

                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });

and another small part of the code '

mAuth2 = FirebaseDatabase.getInstance().getReference();
        mAuth = FirebaseAuth.getInstance();'

I appreciate your help and collaboration